Women who earn less than their male counterparts are more likely to have a major depressive disorder or a generalized anxiety disorder, according to a new study conducted at Columbia University’s Mailman School of Public Health.

The researchers found that women who make less than men with similar skills, experience, and education are 2.5 times more likely to suffer from major depression than men. In contrast, women whose income is equal to or exceeds that of their male counterparts have the same rates of depression as men.

The pattern is similar for generalized anxiety disorder. Overall, women are 2.5 times more likely to have faced anxiety in the past year than men. For women who make less than their male counterparts, the likelihood of experiencing anxiety disorder is four times greater than the likelihood for men. Without the pay gap, women’s chances of suffering from the disorder drop significantly—though they still exceed that of men.

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The study is based on a single year of data, a 2001-2002 survey of U.S.-based working adults, ages 30 to 65.

Jonathan Platt, the study’s co-author and a PhD student in Columbia’s Department of Epidemiology, says he was taken aback by the clarity of the results. “To see such a clear signal emerge, and for it to be so robust in the data, was pretty surprising,” he says.

Platt adds that he and his fellow researchers see the wage gap as a proxy, “a marker for bias and discrimination in the workplace.” It’s not necessarily the smaller paycheck itself that correlates to the increased risk of mood disorders. Rather, it’s likely a complex set of factors, such as the effect of being undervalued and of facing discrimination and limited opportunity.

“Structural forms of discrimination may explain a substantial proportion of gender disparities in mood and anxiety disorders in the U.S. adult population,” said senior author and assistant professor of Epidemiology Katherine Keyes in a press release. “Greater attention to the fundamental mechanisms that perpetuate wage disparities is needed, not only because it is unjust, but so that we may understand and be able to intervene to reduce subsequent health risks and disparities.”

Platt says he believes the study underlines the importance of pursuing policies like paid parental leave, affordable childcare, increased pay transparency and other measures that may help narrow the gap—and hopefully improve women’s mental health.